Recent advances in indoor location systems leverage existing wireless communication infrastructure (e.g., 802.11 and GSM) to provide a value-added location service. The major advantage of these approaches is that a user does not have to purchase any specialized equipment and can still benefit from location-aware computing. Leveraging public infrastructure has many advantages, but one major drawback is that users have very little control of the infrastructure itself. Service providers adjust the operational parameters of WiFi access points and cellular towers with little or no warning. These changes require recalibration of the location system and may result in inaccurate location data until the changes are discovered. An alternative is to introduce new infrastructure in the home by distributing many low-cost, short-range beacons. The time required for installation and the possible impact to home aesthetics, however, may limit adoption.
Indoor positioning has been very active in the ubiquitous computing research community in the subsequent half decade. Several characteristics distinguish different solutions, such as the underlying signaling technology (e.g., IR, RF, load sensing, computer vision or audition), line-of-sight requirements, accuracy, and cost of scaling the solution over space and over number of items. Although we do not intend to provide a complete survey of this topic, we highlight those projects with characteristics most relevant to the motivation for power line positioning, namely the requirements for additional infrastructure and algorithmic approach.
The earliest indoor positioning solutions introduced new infrastructure to support localization. See, for example, “Active Bat.” The BAT Ultrasonic Location System, 2006; O'Connell, T., Jensen, P., Dey, A. K., and Abowd, G. D., “Location in the Aware Home,” Position paper for Workshop on Location Modeling for Ubiquitous Computing at Ubicomp 2001 Sep. 30, Atlanta, Ga., 2001; Priyantha, N. B., Chakraborty, A., and Balakrishnan, H., “The Cricket Location-Support System,” Proceedings of The International Conference on Mobile Computing and Networking (Mobicom 2000), Boston, Mass., August, 2000; and Want, R., Hopper, A., Falcao, V., and Gibbons, J., “The active badge location system,” “ACM Transactions on Information Systems,” Volume 10, pp. 91-102, January, 1992.
Despite some success, as indicated by some commercialized products, the cost and effort of installation are a major drawback to wide-scale deployment, particularly in domestic settings. Thus, many new projects in location-based systems research reuse existing infrastructure to ease the burden of deployment and lower the cost. The earliest demonstrations leveraged 802.11 access points (see, for example, Bahl, P. and Padmanabhan, “V. RADAR: An In-Building RF-Based User Location and Tracking System,” Proceedings of IEEE Infocom, Los Alamitos, pp. 775-784, 2000; Castro, P., Chiu, et al., “A Probabilistic Room Location Service for Wireless Networked Environments,” Proceedings of Ubicomp 2001, pp. 18-34, 2001; and LaMarca, A., et al., “Place Lab: Device Positioning Using Radio Beacons in the Wild,” Proceedings of Pervasive 2005, Munich, Germany, pp. 116-133, 2005. More recent examples explore Bluetooth (see, for example, Madhavapeddy, A. and Tse, T., “Study of Bluetooth Propagation Using Accurate Indoor Location Mapping,” The Seventh International Conference on Ubiquitous Computing (UbiComp 2005), Tokyo, Japan, pp 105-122, September 2005, and wireless telephony infrastructure, such as GSM (see, for example, V. Otsason et al., “Accurate GSM Indoor Localization,” Proceedings of The Seventh International Conference on Ubiquitous Computing (UbiComp 2005), Tokyo, Japan, September, 2005), or FM transmission towers (see, for example, Krumm, J., Cermak, G., and Horvitz, E., “RightSPOT: A Novel Sense of Location for a Smart Personal Object,” Proceedings of Ubicomp 2003, Seattle, Wash., pp. 36-43, 2003. Concerns about system resolution eliminate the FM solution for domestic use.
Another concern we highlighted in the introduction is that individuals and households may not be able to control the characteristics of this infrastructure, resulting in the need to recalibrate if parameters change. The desire to control the infrastructure and to scale inexpensively to track a large number of objects inspired the search for a solution like the power line system presented here.
Traditional wireless signal triangulation, such as 802.11 access point triangulation, uses Received Signal Strength Indicator (RSSI) information to estimate distance and determine a location based on geometric calculations employing the RSSI data. Other techniques include the use of Time of Arrival, as in the case of ultrasound, or Angle of Arrival, such as with Ultra-wideband positioning (see the Ubisense website, for example). Ultrasonic solutions, such as Cricket (see Priyantha, N. B., et al., “The Cricket Location-Support System,” Proceedings of The International Conference on Mobile Computing and Networking (Mobicom 2000), Boston, Mass., August, 2000, and Active Bat, the BAT Ultrasonic Location System, provide precise centimeter resolution, but require line-of-sight operation indoors. Therefore, they require extensive sensor installations for full coverage. Some radio frequency technologies, such as 802.11 triangulation, employ overdetermination of transmitting sites (e.g., wireless access points) to avoid issues of occlusion induced by multipath propagation caused by reflections in the environment.
Fingerprinting of the received signals can help overcome the multipath problem. Fingerprinting improves on other means of estimation by taking into account the effects that buildings, solid objects, or people may have on a wireless or RF signal, such as reflection and attenuation. Fingerprinting works by recording the characteristics of wireless signals at a given position and later inferring that position when the same signature is seen again. A survey of signals over a surveyed space allow for the creation of a map that can be used to relate a signal fingerprint to a location.
Power lines are already in place in most buildings and the power network reaches more homes than either cable systems or telephone lines. Thus, for many years, people have been using power lines in buildings (especially homes) to deliver more than just electricity. Several home automation technologies leverage the power line for communications and control. The most popular example is the X10 control protocol for home automation, a standard that is more than 30 years old and is a very popular, low-cost alternative for homeowners. Over the past decade, there have been a number of efforts to produce power line communications capabilities, driven by industrial consortia such as the HomePlug Power line Alliance, and efforts such as Broadband over Power line (BPL). Because electricity used for power is sent over power lines at a lower frequency (e.g., 60 Hz) than Internet data signals modulated on high frequency carriers, power and data can coexist on the same power line without interference.
It would be desirable to have an indoor location system that takes advantage of existing infrastructure, such as electrical power lines, and the like, and which does not require additional infrastructure.